Tag Archives: talks

Q+ Hangout: Steven Flammia

Here are the details of the next Q+ hangout.

Speaker: Steven Flammia (University of Sydney)

Title: Thermalization, Error-Correction, and Memory Lifetime for Ising Anyon Systems

Abstract:
We consider two-dimensional lattice models that support Ising anyonic excitations and are coupled to a thermal bath, and we propose a phenomenological model to describe the resulting short-time dynamics, including pair-creation, hopping, braiding, and fusion of anyons. By explicitly constructing topological quantum error-correcting codes for this class of system, we use our thermalization model to estimate the lifetime of quantum information stored in the code space. To decode and correct errors in these codes, we adapt several existing topological decoders to the non-Abelian setting: one based on Edmond’s perfect matching algorithm and one based on the renormalization group. These decoders provably run in polynomial time, and one of them has a provable threshold against a simple iid noise model. Using numerical simulations, we find that the error correction thresholds for these codes/decoders are comparable to similar values for the toric code (an Abelian sub-model consisting of a restricted set of allowed anyons). To our knowledge, these are the first threshold results for quantum codes without explicit Pauli algebraic structure. Joint work with Courtney Brell and Simon Burton.

To watch the talk live go to the Q+ page at the appointed hour.

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Q+ Hangout: Bill Wootters

Here are the details of the next Q+ hangout.

Date/time: Tuesday 18th June 2013 2pm BST/UTC+1

Speaker: Bill Wootters (Williams College)

Title: What is the origin of complex probability amplitudes?

Abstract: I begin this presentation with an attempt to explain the origin of probability amplitudes in quantum theory, but the explanation makes sense only if those amplitudes are real. This result provides motivation for studying the real-vector-space variant of quantum theory. I show how a particular model within real-vector-space quantum theory can produce the appearance of complex probability amplitudes. In this model, a special binary subsystem of the universe, called the universal rebit or “ubit,” plays the role of the complex phase factor. In a certain limit the effective theory emerging from the model mimics standard quantum theory, but if we stop short of this limit the model predicts the spontaneous decoherence of isolated systems.

To watch the talk live go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: David Poulin

Here are the details of the next Q+ hangout.

Date/time: Mon. 27th May 2013 3pm BST (UTC+1)

Speaker: David Poulin (University of Sherbrooke)

Title: Tradeoffs Between Thermal and Quantum Fluctuations in 2D Quantum Memories

Abstract: Under a certain set of conditions collectively known as “local topological order”, the low energy spectrum of a system is robust to local perturbations. This has the consequence that quantum information encoded in the degenerate ground state of such a system is stable at zero temperature. On the other hand, the existence of a macroscopic energy barrier between ground states imply that information encoded in the low energy manifold is robust against thermal fluctuations. Here, we demonstrate that for local commuting projector codes, local topological order prohibits the existence of an energy barrier, which shows a tradeoff between robustness to quantum and thermal fluctuations.

To watch the talk live, go to http://gplus.to/qplus at the appointed hour. To keep up to date with the latest news about Q+ hangouts you can follow us on:

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Q+ Hangout: Dietrich Leibfried (NIST)

Here are the details of the next Q+ hangout. This is our “Nobel Prize” lecture. Dietrich is a long time colleague of David Wineland at NIST and will tell us about the latest research from the Ion Storage Group. Please note the unusual start time of 5pm BST(UTC+1)

To join the hangout or watch the livestream go to http://gplus.to/qplus at the appointed hour.

Date: 23rd April 2013 5pm BST(UTC+1)

Speaker: Dietrich Leibfried (NIST)

Title: Towards scalable quantum information processing and quantum simulation with trapped ions

Abstract:
Quantum information processing (QIP) and Quantum Simulation (QS) can potentially provide an exponential speedup for certain problems over the corresponding (known) algorithms on conventional computers. QIP makes use of the counter-intuitive properties of quantum mechanics, like entanglement and the superposition principle (being in more states than one simultaneously). On the way towards a useful QIP device these properties, mostly subject of thought experiments so far, will have to become a practical reality. I will discuss experiments towards Quantum Information Processing (QIP) and Quantum Simulation (QS) with trapped ions. Most requirements for QIP and QS have been demonstrated in this system, with two big challenges remaining: Improving operation fidelity and scaling up to larger numbers of qubits.

The architecture pursued at the Ion Storage Group at NIST is based on quantum information stored in long lived internal (hyperfine) states of the ions. We investigate the use of laser beams and microwave fields to induce both single-qubit rotations and multi-qubit gates mediated by the Coulomb interaction between ions. Moving ions through a multi-zone trap architecture allows for keeping the number of ions per zone small, while sympathetic cooling with a second ion species can remove energy and entropy from the system.

After a brief introduction to these elements, I will present the current status of experiments and some future perspectives for QIP and QS.

This work has been supported by IARPA, DARPA, ARO, ONR, and the NIST Quantum Information Program.

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Q+ Hangout: Ivette Fuentes

Here are the details of the next Q+ hangout.

Date/time: Tue. 26th March 2pmGMT/UTC

Speaker: Ivette Fuentes (University of Nottingham)

Title: Quantum information processing in spacetime

Abstract:

Cutting-edge experiments in quantum communications are reaching regimes where relativistic effects can no longer be neglected. For example, there are advanced plans to use satellites to implement teleportation and quantum cryptographic protocols. Relativistic effects can be expected at these regimes: the Global Positioning System (GPS), which is a system of satellites that is used for time dissemination and navigation, requires relativistic corrections to determine time and positions accurately.

Therefore, it is timely to understand what are the effects of gravity and motion on entanglement and other quantum properties exploited in quantum information.

In this talk I will show that entanglement can be created or degraded by gravity and non-uniform motion. While relativistic effects can degrade the efficiency of teleportation between moving observers, the effects can also be exploited in quantum information. I will show that the relativistic motion of a quantum system can be used to perform quantum gates. Our results, which will impact future space-based experiments, can be demonstrated in table-top experiments using superconducting circuits.

To watch the talk live go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: Fernando G.S.L. Brandão

Here are the details of the next Q+ hangout.

Date/time: Tue. 26th Feb. 2pmGMT/UTC

Speaker: Fernando G.S.L. Brandão (Universidade Federal de Minas Gerais)

Title: Exponential decay of correlations implies area law

Abstract:

Quantum states of many particles are fundamental to our understanding of many-body physics. Yet they are extremely daunting objects, requiring in the worst case an exponential number of parameters in the number of subsystems to be even approximately described. How then can multi-particle states be useful for giving predictions to physical observables? The intuitive explanation is that physically relevant quantum states, defined as the ones appearing in nature, are usually much simpler than generic quantum states. In this talk I will discuss a recent result that gives further justification to this intuition.

The result shows that exponential decay of correlations, a physically motivated restriction on the set of multi-particle quantum states, implies an area law for the entanglement entropy of systems defined on a line, and thus also an efficient classical description for such systems. The result can be seen as a rigorous justification to the intuition that states with exponential decay of correlations, usually associated with non-critical phases of matter, are simple to describe.

I will outline the main ideas in the proof, that relies on several previous tools from quantum information theory, such as the state merging protocol, and that can also be seen as providing a limitation to the phenomenon of data hiding in quantum states.

Based on arXiv:1206.2947, joint work with Michal Horodecki.

To watch the talk live go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: Roger Colbeck

Here are the details of the next Q+ hangout.

Date: Tuesday 29th January 2013

Time: 2pm GMT/UTC

Speaker: Roger Colbeck (ETH Zurich)

Title: No extension of quantum theory can have improved predictive power

Abstract:

According to quantum theory, measurements generate random outcomes, in stark contrast with classical mechanics. This raises the important question of whether there could exist an extension of the theory which removes this indeterminism, as famously suspected by Einstein, Podolsky and Rosen. Under the assumption of free choice within a particular causal structure, Bell’s work showed this to be impossible. However, existing results do not imply that the current theory is maximally informative. Could it be that certain hidden variable theories (for example) allow us to make more accurate predictions about the outcomes?

In this talk, I will discuss this question and show that, under the same free choice assumption, the answer is negative: no extension of quantum theory can give more information about the outcomes of future measurements than quantum theory itself.

I will then show that as a corollary of this result, we can reach the same conclusion as Pusey, Barrett and Rudolph that the wavefunction cannot be thought of as subjective.

(This is based on arXiv:1005.5173, arXiv:1111.6597 and arXiv:1208.4123)

To watch the talk live go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: Rob Spekkens

Here are the details of the next Q+ hangout.

Date/time: Tuesday 20th November 2pm GMT/UTC

Speaker: Rob Spekkens (Perimeter Institute)

Title: Quantum correlations from the perspective of causal discovery algorithms

Abstract: If correlation does not imply causation, then what does? The beginning of a rigorous answer to this question has been provided by researchers in machine learning, who have developed causal discovery algorithms. These take as their input facts about correlations among a set of observed variables and return as their output a causal structure relating these variables. We show that any attempt to provide a causal explanation of Bell-inequality-violating correlations must contradict a core principle of these algorithms, namely, that an observed statistical independence between variables should not be explained by fine-tuning of the causal parameters. In particular, we demonstrate the need for such fine-tuning for most of the causal mechanisms that have been proposed to underlie Bell correlations, including superluminal causal influences, superdeterminism (that is, a denial of freedom of choice of settings), and retrocausal influences which do not introduce causal cycles. This work suggests a novel perspective on the assumptions underlying Bell’s theorem: the nebulous assumption of “realism” is replaced with the principle that all correlations ought to be explained causally, and Bell’s notion of local causality is replaced with the assumption of no fine-tuning. Finally, we discuss the possibility of avoiding the fine-tuning by replacing conditional probabilities with a noncommutative generalization thereof.

Based on arXiv:1208.4119.

Joint work with Chris Wood.

To watch the talk live, go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: Stephanie Wehner

Here are the details for the next Q+ hangout.

Date: 25th September 2012

Time: 10am British Summer Time

Speaker: Stephanie Wehner (Singapore)

Title: Uncertainty and the generation of randomness

Abstract: We consider the creation of classical randomness by making measurements on a largely unknown quantum system. We show a close relations between measurements that are good at this task and measurements that satisfy strong uncertainty relations. Finally, we mention an application to quantum cryptography.

Please note the unusual start time.

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: Francesco Buscemi

Here are the details for the next Q+ hangout.

Date: 28th August 2012

Time: 2pm British Summer Time

Speaker: Francesco Buscemi (Nagoya University)

Title: All entangled quantum states are nonlocal: equivalence between locality and separability in quantum theory

Abstract:

In this talk I will show how, by slightly modifying the rules of nonlocal games, one can prove that all entangled states violate local realism.

As it is well known, Bell inequalities, which are used to test the violation of local realism, can be equivalently reformulated in terms of nonlocal games (namely, cooperative games with incomplete information) played between one referee and two (or more) players, these latter being separated so to make any form of communication between them impossible during the game. Quantum nonlocality is that property of quantum states that allows players sharing them to win nonlocal games more frequently than the assumption of local realism would imply.

However, as Werner proved in 1989, not all quantum states enable such a violation of local realism. In particular, Werner showed the existence of quantum states that cannot be created locally (the so-called “entangled” states) and, yet, do not allow any violation of local realism in nonlocal games. This fact has been since then considered an unsatisfactory gap in the theory, attracting a considerable amount of attentions in the literature.

In this talk I will present a simple proof of the fact that all entangled states indeed violate local realism. This will be done by considering a new larger class of nonlocal games, which I call “semiquantum,” differing from the old ones merely in that the referee can now communicate with the players through quantum channels, rather than being restricted to use classical ones, as it was tacitly assumed before. I will then prove that one quantum state always provides better payoffs than another quantum state, in semiquantum nonlocal games, if and only if the latter can be obtained from the former, by local operations and shared randomness (LOSR). The main claim will then follow as a corollary.

The new approach not only provides a clear theoretical picture of the relation between locality and separability, but also suggests, thanks to its simplicity, new experimental tests able in principle to verify the violation of local realism in situations where previous experiments would fail.

Based on http://arxiv.org/abs/1106.6095

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

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